This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-334599, filed Nov. 25, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to an electron gun assembly for a color cathode ray tube, and more particularly to a color cathode ray tube provided with an electron gun assembly having a main electron lens of large aperture.
Generally, a color cathode ray tube has an external enclosure or envelope composed of a panel 11 and a funnel 12 integrally joined to the panel 11 as shown in FIG. 1. On the inside face of the panel 11, a phosphor screen or target 13 composed of a stripe-like or dot-like three-color phosphor layers that emits blue light rays, green light rays, and red light rays are formed. Inside the phosphor screen 13, a shadow mask 14 in which a large number of apertures have been made is provided in such a manner that the mask faces the screen. On the other hand, in the neck 15 of the funnel 12, an electron gun assembly 17 is provided which emits three electron beams 16B, 16G, and 16R. The three electron beams 16B, 16G, and 16R emitted from the electron gun assembly 17 are deflected by a horizontal and a vertical deflecting magnetic field generated by a deflection yoke 18 provided on the outside of the funnel 12 and are directed via the shadow mask 14 toward the phosphor screen 13. The phosphor screen 13 is scanned horizontally and vertically by the electron beams, thereby displaying a color image.
In recent years, there have been strong demands for higher resolution of color images. The spot diameter of an electron beam formed on the phosphor screen 13 is considered to be a major factor that determines resolution. The electron beam spot diameter is generally determined by the focusing capability of the electron gun assembly.
The focusing capability is generally determined by the diameter of the main electron lens, the hypothetical object point diameter, the magnification, and others. Specifically, the larger the diameter of the main electron lens is, the smaller the hypothetical object point diameter is, or the lower the magnification is, the smaller the electron beam spot diameter becomes, which improves the resolution.
In a conventional electron gun assembly, for example, the electron gun assembly disclosed in, for example, U.S. Pat. No. 4,712,043, Jpn. Pat. Appln. KOKAI Publication No. 8-22780, or Jpn. Pat. Appln. KOKAI Publication No. 9-320485, an intermediate electrode to which about an intermediate potential higher than the focus voltage and lower than the anode voltage is supplied is provided between the focus electrode and anode electrode. In the respective opposite faces, an opening with an elliptical section long in the in-line direction has been made so as to be common to three electron beams.
In the electron gun assembly having such a configuration, the main electron lens of large aperture has been formed by forming not only an expanded electric field in the direction of electron beam advance but also a continuous electric field in the in-line direction. With the electron gun assembly, the main electron lens of large aperture makes smaller the electron beam spot converged on the screen, realizing a high resolution.
In the electron gun assembly with such a configuration, however, the electrode in which an opening with an elliptical section long in the in-line direction has been made so as to be common to three electron beams causes side beams to converge, with great halos occurring in the direction of the center beam as shown in FIG. 2. To avoid this phenomenon, one approach is to design an electrode structure in designing an electron gun assembly so that side beams may be bent toward the center beam beforehand so as to enter the main electron lens of large aperture at an angle. With the electrode structure thus designed, side beams are caused to enter the main electron lens at an angle, with the result that the side beams pass through the parts where the potential distribution is relatively uniform closer to the center beam within the main electron lens. As a result of the side beams entering the main electron lens at an angle, its spherical aberration is increased, which is balanced against the spherical aberration occurred on the opposite side, with the result that the side beams are prevented from being converged in a state where great halos appear in the direction of the center beam as shown in FIG. 2.
However, with the electrode structure that causes side beams to enter the main electron lens at an angle, since side beams are bent before they enter the main electron lens, the center of the side beam through-hole, for example, the center of the side beam through-hole in each of the second grid and third grid, or the center of a sub-lens composed of the third, fourth, and fifth grids, has been offset.
In the former case where the center of the side beam through-hole in each of the second and third grids has been offset, when the side beams are bent toward the center beam, an aberration occurs in the side beams between the second grid and third grid, regardless of the great potential difference, because the diameter of the opening is small. This causes the problem of the side beams being distorted seriously. In the latter case where the center of the sub-lens composed of the third, fourth, and fifth grids has been offset, the shape of inner core pins necessary to assemble the electrodes constituting an electron gun assembly must be made complex, which causes the problem of making errors more liable to occur in assembly.
In the main electron lens of large aperture described above, since the shape of the opening between the electrodes is of an elliptical section long in the horizontal direction, the lens diameter in the vertical direction is much smaller than that in the horizontal direction, causing the electron beam spot on the screen to be converged excessively in the vertical direction and insufficiently in the horizontal direction. To overcome this problem, an electric field correcting electrode plate is provided in a position back from the opening in the focus electrode. Each hole corresponding to each of the three electron beams made in the electric field correcting electrode plate is formed into a shape with its height much greater than its width.
As described above, the horizontal diameter of each of the holes corresponding to the respective three electron beams is set small, which corrects insufficient convergence in the horizontal direction and excessive convergence in the vertical direction. However, as a result of the horizontal diameter of each of the holes corresponding to the respective three electron beams being set small, when the electron beams pass through the holes, the holes give local aberrations to the electron beams. This impairs seriously the original effect of the main electron lens of large aperture produced by expanding the lens electric field in the horizontal direction and in the direction of electron beam advance and thereby forming the main electron lens.
Furthermore, there is a limit to the length of the intermediate electrode forming the main electron lens of large aperture. When the intermediate electrode is too long, the lens electric field is divided, practically forming separate electric field lenses between the fifth grid and intermediate electrode GM and between the intermediate electrode GM and the sixth electrode G6 as shown in FIG. 3B, which increases the lens aberration. As a result, the electron beam spot diameter becomes larger, degrading the resolution.
The object of the present invention is to provide an electron gun assembly which not only makes the aperture of the main electron lens larger while alleviating the aberration in the main electron lens, but also assures a high assembly accuracy and a good image characteristic all over the screen.
According to the present invention, there is provided a color cathode ray tube comprising: an electron gun assembly including an electron beam generating section for emitting and forming three electron beams in in-line arrangement and a main electron lens section for converging the electron beams on a screen; and a deflection yoke for generating a deflection magnetic field to deflect the election beams emitted from the electron gun assembly in the horizontal and vertical directions for scanning on a screen, wherein the main convergence lens of the electron gun assembly includes a focus electrode to which a medium focus voltage is applied, an anode electrode to which a high anode voltage is applied, and at least one intermediate electrode which is provided between the focus electrode and anode electrode and to which a medium high intermediate potential is applied, the intermediate potential being higher than the medium focus voltage and lower than the high anode voltage and obtained by dividing the high anode potential with a resistor provided near the electron gun assembly, and the opening sections of the anode electrode and intermediate electrode adjacent to each other are each a cylindrical unit long in the in-line direction and common to the three electron beams and have a multiple pole lens provided between the anode electrode and intermediate electrode adjacent to each other, the multiple pole lens acting equally on the three electron beams and diverging them relatively in the vertical direction and converging them relatively in the horizontal direction.
Furthermore, the color cathode ray tube is characterized in that the opening sections of the anode electrode and intermediate electrode adjacent to each other are each a cylindrical unit long in the in-line direction and common to the three electron beams, with the diameter of the openings in the direction crossing at right angles with the in-line direction of the cylindrical units being such that the diameter of the opening in the anode electrode is set smaller than that of the opening in the intermediate electrode to form a multiple pole lens common to the three electron beams.
FIGS. 4A, 4B, and 4C show the potential distribution in a conventional main electron lens of large aperture, a graph of a quadratic differential of the potential on the tube axis, and the trajectories of side beams in the main electron lens of large aperture. FIGS. 5A, 5B, and 5C show the potential distribution in a main electron lens of large aperture according to the present invention, a graph of a quadratic differential of the potential on the tube axis, and the trajectories of side beams in the main electron lens of large aperture. In FIGS. 4A and 4C and FIGS. 5B and 5C, the fifth grid corresponds to the focus electrode G5 and the sixth grid G6 corresponds to the anode electrode. Between the fifth and sixth grids G5, G6, the intermediate electrode GM is provided.
FIG. 4A and FIG. 5A schematically show a potential distribution occurring in the conventional main electron lens of large aperture and that of the present invention, respectively. As seen from FIG. 4A and FIG. 5A, the anode electrode G6 and intermediate electrode GM adjacent to each other in the color cathode ray tube of the present invention are each a cylindrical unit long in the in-line direction and common to three electron beams. As for the diameter of the openings in the direction crossing at right angles with the in-line direction (horizontal direction), the diameter of the opening in the anode electrode is set smaller than that of the opening in the intermediate electrode.
With such a configuration, a multiple pole lens that acts equally on three electron beams and converges them relatively in the horizontal direction and diverges them relatively in the vertical direction is formed between the anode electrode G6 and intermediate electrode GM adjacent to each other. As a result, the electric field penetrating into the intermediate electrode is pressed by the face of the anode electrode facing the intermediate electrode, making the potential in the intermediate electrode denser than in the prior art. Consequently, the lens between the focus electrode G5 and intermediate electrode GM and the lens between the intermediate electrode GM and anode electrode G6 can be easily formed as a continuous single lens as compared with an electronic lens system using a conventional electrode structure. In the prior art (as disclosed in Japanese Patent Application No. 11-131469), to connect two lenses formed in front of and behind the intermediate electrode to form a continuous single lens of large aperture, the length L of the intermediate electrode in the direction of electron beam advance should be restricted by the short diameter DV (the diameter in the vertical direction) of the opening in front of and behind the intermediate electrode, where DV satisfies the following expression:
0.3xe2x89xa6DV/L 23 0.6
With the configuration of the present invention, however, the density of the electrical potential is increased in the intermediate electrodes so that the two consecutive lenses in front of and behind the intermediate electrode can be easily connected to each other, which make it possible to make greater the length of the intermediate electrode in the direction of electron beam advance, without cutting off the connection.
FIG. 4B and FIG. 5B show graphs of the state of potential (Vo) changes as a result of a quadratic differential of the potential (Voxe2x80x3) on the tube axis in the prior art and in the present invention, respectively. The graphs of a quadratic differential of the potential on the tube axis show convergence regions and divergence region in the main electron lens of large aperture. In the conventional main electron lens of large aperture in FIG. 4B, a quadratic differential of the potential on the tube axis changes from a convergence region to a divergence region in the direction of electron beam advance. Near the midpoint, however, the main electron lens becomes such a lens as alternates between a divergence region and a convergence region, with the result that the main electron lens becomes a lens which effects convergence, divergence, convergence, and divergence in that order. A lens system that alternates between convergence and divergence this way is considered to be undesirable because it increases the lens aberration. In contrast, a quadratic differential of the potential on the tube axis according to the present invention changes from a convergence region to a divergence region in the direction of electron beam advance. Although the quadratic differential changes a little up and down near the midpoint, all the changes take place in convergence regions, with the result that the main electron lens becomes a lens with only one set of convergence and divergence. As a result, the main electron lens of large aperture according to the present invention can progressively prevents the lens aberration from increasing when the length of the intermediate electrode is increased, as compared with the conventional main electron lens of large aperture. The observation of a quadratic differential of the potential on the axis has shown that the divergence region rises sharply. This is because the bump (concave) in the intermediate part has shifted toward the convergence side as compared with that in the prior art, and the lens effect has increased in the divergence region to maintain a balance as a lens. It is assumed that such a sharp rise in the divergence region become to have the effect of offsetting the aberration occurred in the convergence region, with the result that the aperture of the lens becomes large.
FIG. 4C and FIG. 5C show the trajectories of side beams in the main electron lens of large aperture in the prior art and in the present invention, respectively. Specifically, with the conventional electrode structure, to remove the halo components from the side beams and concentrate the three electron beams on the screen, the side beams have to be bent toward the center beam before entering the main electron lens of large aperture. Because of this, the center of a side beam through-hole, for example, the center of each of the side beam through-holes of the second and third grids, or the center of the sub-lens composed of the third, fourth, and fifth grids, has been designed to be offset. With the center of the side beam through-hole of each of the second and third grids being offset, since the diameter of the aperture is small for the large potential difference between the second and third grids, an aberration occurs when the side beams are bent toward the center beam, with the result that the side beams are distorted seriously. In addition, when the center of the sub-lens composed of the third, fourth, and fifth grids has been offset, this makes it necessary to complicate the shape of the inner core pin needed in assembling the electrodes constituting an electron gun assembly, which causes the problem of making errors liable to occur in assembly.
With the present invention, however, since the main electron lens of large aperture has the function of bending the side beams toward the center beam positively, the side beams have only to be bent a little toward the center beam before entering the main electron lens of large aperture, or there is no need to bend the side beams toward the center beam. Consequently, the aberration occurred in bending the side bean toward the center beam between the second and third grids is alleviated (or eliminated). Furthermore, this provides the advantage of making it unnecessary to complicate the shape of the inner core pin needed in assembling the electrodes constituting an electron gun assembly.
On the other hand, in the conventional main electron lens of large aperture, since the opening between the electrodes has an elliptical shape in section long in the horizontal direction, the diameter of the lens in the vertical direction is much smaller than the diameter of the lens in the horizontal direction, with the result that the electron beam spot on the screen converges excessively in the vertical direction and insufficiently in the horizontal direction. To correct this phenomenon, an electric field correcting electrode plate is provided in a position back from the opening of the focus electrode. Holes made separately for three electron beams in the electric field correcting electrode plate are made extremely long vertically for their width. In this way, making the diameter of the holes formed for the respective three electron beams is made smaller in the horizontal direction corrects insufficient convergence in the horizontal direction and excessive convergence in the vertical direction. As a result of making smaller the horizontal diameter of the holes formed for the respective three electron beams, local aberration components are given at the holes when the electron beams pass through the holes. This causes the lens electric field to expand in the horizontal direction and in the direction of electron beam advance, which impairs seriously the effect of forming the main electron lens of large aperture. With the present invention, however, since the convergence lens components, which are common to three electron beams and diverge relatively in the vertical direction and converge relatively in the horizontal direction, lie between the anode electrode G6 and intermediate electrode GM. Therefore, it is not necessary to make extremely small the horizontal diameter of the holes made in the electric field correcting electrode plate provided in the position back from the opening in the focus electrode, thereby alleviating the local aberration components at the holes made for the respective three electron beams in the electric field correcting electrode plate provided in the position back from the opening of the focus electrode.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.